Automatic control system for boiler or steam generator



June 26, 1956 A. CLARKSON El'AL 2,751,894

AUTOMATIC CONTROL SYSTEM FOR BOILER OR STEAM GENERATOR Filed May 1, 1951 4 Sheets-Sheet 1 INVEN T 0R5.

I OIL TA lwc June 26, 1956 A. CLARKSON ETAL 2,751,894

AUTOMATIC CONTROL SYSTEM FOR BOILER OR STEAM GENERATOR Filed May 1 1951 4 Sheets-Sheet 2 iii? 121 3 126a INVENTORS.

ZZ- Zara/5072, By gm d wmzel June 26, 1956 A. CLARKSON ETAL 2,751,894

AUTOMATIC CONTROL SYSTEM FOR BOILER OR STEAM GENERATOR Filed May 1, 1951 4 Sheets-Sheet 3 A ig f United States Patent AUTOMATIC CONTROL SYSTEM FOR BOILER OR STEAM GENERATOR Alick Clarkson, Pauls Spur, Ariz., and Donald W. Miller, Elmhurst, Ill., assignors of one-half to Vapor Heating Corporation, Chicago, 11]., a corporation of Delaware Application May 1, 1951, Serial No. 224,026

8 Claims. (Cl. 122-448) This invention relates to improvements in automatic control systems for boilers and steam generators.

The primary object of the invention is to provide an improved method and apparatus for automatically controlling the rate at which fuel, or fuel and air, is supplied to the combustion chamber of a boiler, steam generator or other analogous equipment, in relation to the rate at which water or other liquid, to be heated or vaporized, is fed to the boiler. The invention, in this connection, contemplates the provision of an apparatus adapted to proportionately vary the delivery of fuel and combustion air in relation to each other to the fire chamber of a boiler and also in relation to the rate of flow of feed water or other liquid delivered to the boiler.

A more specific object is to provide improved means which will function immediately upon accelerated or decelerated flow of water or other liquid to a boiler to effect corresponding and commensurate acceleration or deceleration of the delivery of fuel and air to the fire chamber; the apparatus being designed to maintain a fixed rate of fuel and air delivery as long as the flow rate of said water or other liquid to the boiler is maintained constant.

The underlying purpose of the improved method and apparatus is to control the operation of a boiler in relation to its intake rather than by its temperature or output. To this end the invention may be described briefly as comprising a means positioned in the path of movement of the feed water or other liquid to the boiler and is responsive to the momentary rate at which the water or liquid is supplied to the boiler. This means is operatively connected to a valve for controlling the delivery of pressure fluid, for example fuel oil to a hydraulic motor adapted to operate to open and close valves for controlling the admission of fuel and air to the fire chamber of the boiler, the valve for controlling the admission of pressure liquid to the hydraulic motor being such that it will respond immediately to any accelerated or decelerated flow of liquid to the boiler and when the said acceleration or deceleration of said liquid ceases and the liquid flow becomes constant the valve functions to shut off the supply of pressure fluid to the hydraulic motor and maintains the said fuel and air valves in a position to deliver a constant volume of fuel and air to the combustion chamber. The arrangements and constructions of the parts are such that the force required to actuate the fuel and air valves is performed by the hydraulic motor. Consequently, the operation of these valves does not impose a load on the flow responsive element. Thus, the

velocity-responsive element always works against a definite predetermined load and is uninfluenced by any variations in the amount of force and work required to actuate the fuel and air control valves.

The novel features of the invention and the new prin- I ciples involved can best be explained concurrently with the ensuing detailed description and by reference to the accompanying drawings, wherein: 4

Fig. l is a schematic diagram of a steam generating system incorporating a preferred embodiment of the subject invention together with the major components of the system;

Fig. 2 is a perspective view, largely schematic, intended as an aid to comprehension of the new control unit and its operating principles as distinguished from an exact portrayal of the mechanism as actually constructed-cerrain parts having been omitted from this View in order to avoid masking other parts which cannot otherwise be so readily illustrated;

Fig. 2a is a retarder device forming a part of the improved control unit;

Fig. 3 is a plan view of the control device;

Fig. 4 is a sectional view taken at line 44 of Fig. 3 and shown on a slightly larger scale for purpose of clearness of illustration;

Figs. 5, 6 and 7 are a series of enlarged sections taken at line 55 of Fig. 4 to show different positions of a valve structure shown in Fig. 4.

Fig. 8 is a front elevation as viewed at line 88 of Fig. 3;

Fig. 9 is a fragmentary view taken at line 9-9 of Fig. 3; and

Fig. 10 is a fragmentary View taken at line 1010 of Fig. 3.

The schematic diagram of Fig. 1 pertains to a steam generating system and includes a number of more or less conventional appurtenances embodied in the system and affected by the operation of the control unit and which also serve to complete the flow path through the generator.

The focal center of the system is, of course, the boiler or steam generator per se, which is identified as a whole by reference numeral 10. This is a known type of boiler which is adapted to function either as a hot water boiler or as a steam generator. It comprises several concentric groups of coiled tubingwhich groups are. connected together in series and identified, respectively, by reference numerals 12-15 inclusive. The space 16 embraced by the coils 12 constitutes a heating chamber, above which is disposed the fire chamber 17, a fuel burner surmounted by a spray head or nozzle 18 which in the case of liquid fuel functions as an atomizer.

Water is supplied to the boiler 10 from a reservoir 20; the water being withdrawn from the reservoir and forced through the several coils of the boiler by means of pump 21. The course of the feed water from the reservoir is via conduit 22, pump 21, conduit 23, the improved control unit 24 of the present invention, conduit 25, preheater 26 and conduit 27 to the intake end of the outer tubing group 15, A check valve 28 prevents reverse flow in event of failure of the pump. Steam and hot water emerge from the generator by way of a conduit 30 and pass to a steam separator 31 wherein the residue of hot water is removed for return to the reservoir 20 via a conduit 32 which is connected through a bucket water trap 33 and conduit 34 to the heating coil 35 of the feed water preheater 26, The discharge end of the coil 35 is connected to a conduit 36 leading to the water reservoir 20. It should be clear that hot water from the separator 31, on its way back to the reservoir, serves to preheat the feed water which is passed through the preheater 26. Steam is taken from the separator 31 through a conduit 38 which includes a shut-off valve 39.

A conduit 40 connects the upper end of the steam' separator with the actuating cylinder of a steam pressure controlled valve structure 41 including a valve element,

41 for controlling a bleed port 42 which, when the valve element 41 is open, communicates with a conduit 42 which leads to the feed water conduit 23. When the steam'pressure in the separator 31 rises above a predetermined maximum the valve structure operates to conduit I evident that as the "decreases the supply of. fuel. 7 V on the valve steni51 is relieved, the valve 51 is moved upwardly by the spring 51 to increase the effective openopen valve element 41* to divert feed water from the if desired, the water diverted may be returned to the reservoir 2% through a conduit 43, or

. otherwise disposedof. The pump 21 has a constant output. I-Ience,the volume of water delivered tothe boiler via conduits 25 and 27 per unit of time is a function of the quantity diverted via conduit 42 wl1ich, in turn, is a function of the extent to which the valv e element 41" is open. The actuating mechanism forming a part of saidvalve structure is so loaded, spring or otherwise,

that the opening of valve element t is proportional to the pressure within the boiler, thereby reducing the volume of water delivered through the control unit 24 and thereby effects a proportional reduction in the delivery of fuel to reduce the Heating effect in the boiler. It will be sure required to open valve element 41 m delivery of feed water to control element 24 is reduced and vice versa.

Any large and abrupt demand fof steam obviously will cause a correspondingly sharp decrease of steam pressure andfa consequent sharp increase in the rate at which water is fed to the boiler; and sincethe boiler is of the,

the momentary rate at which feed water is being supplied to the boiler and which is capable of instantly and accurately regulating the-supply of fueland air to the burner;

It is now in order to regulate the supply of fuel, whether gas or oil, to the spray head or nozile 18 and thus to the burner 17, and at the same time to regulate the forced draft to burner 17 whereby to maintain a correct balance between fuel and air.

boiler pressure rises above the pres describe the flow-responsive con- 7 'trol unit 24, the function of which is to'automatical'ly 4. divided into two chambers 57 and 58 by a flexible diaphragm 59 the peripheral margin of which is clamped between theopposing annular faces of the two body castings. Casting 53 has a feed inlet port 60, and casting 54 has a feed Water outlet port 61 leading to the boiler through conduit 25. The diaphragm 59 has an axial opening through which is inserted a shouldered bushing 63 having a central opening 6? constituting an orifice through which feed water flows from chamber 57 to chamber 582 A tapered pin 66 extends axially through the orifice 64' and thus constricts said' orifice to an extent dependent upon the position of said pin, axis'wise, relatively to the diaphragm; The end portion 66' of said pin is threaded into a plug 67 and secured by a locknut 68. It will be self-evident that the effective area of the orifice 64 can be varied by adjusting the position of pin 66 axiswise. This adjustment is carried out at the factory and,l9rdi narily,- is intended to be of a permanent nature. Manifestly, Whenever feed water is passing through the unit the diaphragm 59 will yield proportionately to the resultant hydraulic pressure difierential as between the two chambers 57 and-58. However, it is not ordinarily feasible to connect'the' fuel control valve and the air" valve directly to the diaphragm because the force required to actuate those-elements and the'iriterconnecting linkage will necssarily vary to' a considerable extent; and any variation in the load imposed'upon the diaphragm would, of course; be reflected in a corresponding variation in its response to pressure changes. Hence, in order to maintain accurate control over a long period of time and under the conditions which usually prevail, it is necessary to take care that the load or resistance against which the diaphragm 59 operates is maintained constant insofar as practicability will permit 7 V IAnarm or pitrnan 71, best shown in Fig. 2, is connected at one end to the diaphragm 5 9 and pivotally connected at 72 tothe near end of a lever arrn 73, keyed or otherwise fixedly secured to a shaft'74 journaled in of attachment for the inner end of a spiral spring 81,

Fuel, usually oil or gas, is supplied to the sprayhead or nozzle 18 through a conduit in which is included a valve 51 which controls the rate of fuel supply to the burner arid, in film, is automatically controlled by the control unit 24 iii harmony with the rate at which feed water is momentarily being supplied to the boiler. In

' order to p'rbgiesSiveIy vary the amount of fuel delivered bythe valve 51, tl1e.valv is preferably providedwith a spring closed yalv'e 5 1 having a V -slot 51 formed there in (Fig. 8) and communicating with the outlet port 51 for delivering the fuel to the c'onduit 50 leading to the spray head or some 18. When the valve 51 is depressed, as hereiiiafter described, the V-slot 51 is positioned to decrease the ifectiye valve opening and thereby pressed air orother fluid pressure is delivered into the spray head through conduit 50 to'provide the pressure for atomiZing the oil. The volume of air supplied to the burner 17, in the form of a forceddraft from a blower 52, is cofitrolled by a damper 52 (Fig. 1) 'Which, like- Wis'e, is automatically controlled, in mm, by the control unit 24 in harmony with the momentary feed water supply rate and, incidentally, the momentary fuel supply rat'eJ a V The central unit 24 is illustrated in detail in Figs; 24,0 inclusive and comprises-several. suitably formed body castings identified by reference numerals 53-56 inclusive. said body :astings, conjointly,' forrni ng a self-con.-

to form a housing'having an interior cavity which is When downward pressure the outer end; of which is anchored to a cup 32 centeredinthe bore 83 of body casting 55. The flange of cup 82 has several circumferentially spaced notches adapted to register selectively with a loclgingscrew SS, WhlCl'1 l(.l S the cup against rotation. The cup 82 is capable of being rotated so as to tension the springs and then locked against further rotation by means of screw 85. Ndw it will be apparent that the diaphragm 59 is bisedby spring 31, which seryes as a restoring spring for the diaphragm and atfofds a definite load against whichjhe diaphragm operates. h V

A second shaft 86, co-axial with shaft '74, is drivably connected-to the latter through a coupling 37 and iournaled in the bore of a third shaft 33, which latter has an integral arm 82m the distal end of which is attached a crank pin Qtl. Shaft 33 is journaled in a stationary sleeve 92 which is pressed into a central bore of body casting 5 6,- the latter being detaehably secured tobody casting 55'by means of machine screws %'and serving as a support for sleeve 92 and various other t asshown in Figsz 5,-6 and 7, to formal rotary valve element of generally rectangular cross-section, identified by reference numeral 36% This valve element closely.

municates with an outlet slot 109 formed in the shaft 8d and leading into a crank chamber 101,- the latter of bushings 75 and 76 and extending through a water-tight An intermediate area of shaft 86 is milled on two sides,

which is connected through bore 102 with an outlet conduit 109 (Figs. 1, 2 and 4) which returns the discharged fluid to the oil tank 107. Another set of ports arranged in quadrature to the above-mentioned ports are identified by reference numerals 104 to 105 inclusive. A pipe line 108see Fig. 1is connected to a pump 108 and supplies a fluid under pressure to the pipe 99. Any suitable pressure fluid may be used. However, inasmuch as the present system includes the use of oil as a fuel, the pump 105s is the oil pump for feeding oil under pressure to the spray head or nozzle 18. The ports 104 and 105 are connected via conduits 110 and 111 to a hydraulic motor in the form of two cylinders 112 and 113 and a pair of pistons operatively positioned therein. One of the pistons is designated 114 and is reciprocable in cylinder 112. The other piston is designated 115 and is reciprocable in cylinder 113. These pistons are connected by pitmans 116 and 117 to the crank pin 90. Suitable labyrinths 119 and 119Fig. 4serve to prevent fluid leakage along shaft 86.

When the diaphragm 59 is subjected to a pressure differential it yields proportionately and produces a corresponding rotation of shafts 74 and 86. Thus, the valve element 86*, which is an integral part of shaft 86, is rotated proportionately to the deflection of the diaphragm, as illustrated in Figs. 6 and 7. Upon inspection of Fig. 6, which represents a condition obtaining immediately following an initiation of feed water flow, it will be seen that oil pressure is communicated from the inlet port 97 to the port 105 and thence via conduit 111 to cylinder 113. At the same time port 104 is put into communication with port 96 and outlet channel 100 thereby establishing a path for discharge of oil from cylinder 112. The resultant forward movement of piston 115 and retractive movement of piston 114 is communicated via pitmans 116 and 117, crank pin 90 and arm 89 to shaft 88, which rotates in the same direction as shaft 86 previously rotated. When shaft 88 has rotated to such an extent that its ports 95 and 96 are again closed by valve element 86 as depicted in Fig. 7, its rotation is halted by reason of the fact that the flow of oil to cylinder 113 is cut off. The point to be noted is that shaft 88 promptly follows the rotation of shaft 86 and is always of the same angularity, and that the force expended in rotating shaft 88 is not derived from shaft 36 but, instead, from a separate source; in this case an oil pump 1055*. Consequently, any variation in the load imposed upon shaft 88 is not reflected in the load upon diaphragm 59. If then the rotations of shaft 88 are utilized to control the fuel supply valve 51 and the damper 52, any variations in the amount of force required to move those elements will not influence the response of diaphragm 59 and will not influence the extent of movement of either controlled element because the hydraulic drive has the capacity to develop all the torque that could possibly be needed.

Whenever feed water is flowing the valve element 86 will be in an off-normal position such, for example, as that of Figs. 6 and 7 and will retain that position until a change occurs in the rate of flow. An increase in the rate of flow will cause further rotation of elements 86 and 88 in the clockwise direction, as viewed in Figs. 6 and 7; and a decrease in the rate of flow obviously will cause the same two elements to rotate in the opposite direction to an extent corresponding to-the diminished deflection of the diaphragm. For example, if the flow of feed water decreases slightly the valve 86 is moved in a clockwise direction to open ports 95 and 96, thereby putting the oil supply port in communication with. port 104. In such case the ports 105 and 96 are put into communication with each other to establish a passage whereby the oil may discharge from cylinder 113 and ports 95 and 104 are put into communication, whereby oil under pressure is delivered into cylinder 112 until the inward movement of piston 114 .and the consequent rotation of shaft 88 moves the ports and 96 until they are closed by valve 86*. Should the flow of feed Water cease altogether the diaphragm will return to its normal state as determined by restoring spring 81. In such case the valve 86 will assume the vertical position shown in Fig. 5 and the ports 95 and 96, wherever they may be at that time, will be moved by the pressure fluid in cylinder 112 until these ports assume the closed position shown in Fig. 5. Also cam 131 which rotates with shaft 88 (Figs. 1, 4 and 8), rocks a lever 132 to actuate a push button 133 of a snap switch 134 and thereby open an electrical energizing circuit through solenoid valve 135 and thereby shut off all delivery of fuel to the spray or nozzle 18 (Fig. 1).

In order to transmit motion from the shaft 88 to the fuel valve 51 and the air damper in a manner to accurately proportion the amount of fuel and air deliveredto the fire chamber in relation to any given volume of feed water passing through diaphragm orifice 64, the said fuel valve and air damper are operated by means of a pair of adjustable cam plates identified in the drawings by reference numerals and 121. The cam 120 is arranged to operate the fuel supply valve 51 and the cam 121 is arranged to operate the damper 52. Said cam plates are supported on oppositely extending radial arms 122 and 123 and are adjustably distorted by means of suitable adjusting screws 124 so that the cam surfaces 120 and 121 are of such appropriate angularity as to bring about the precisely correct degree of actuation of the valve and damper, respectively for each angular displacement of shaft 38. in other words, the cam plates are adjusted to actuate the fuel supply valve and damper in harmony with the momentary deflections of diaphragm 59 and thus in harmony with the momentary rate of feed water flow. After the cam plates have been adjusted, housing elements 122 and 123a are positioned over the head ends of the screws 124 and bolted in position on the radial arms 122 and 123 so as to prevent the making of unauthorized changes in the cam adjustments.

The fuel supply valve 51 is provided with an actuating arm 125 which carries a cam follower in the form of a roller 125 bearing against the cam surface 120 of cam plate 120. The actuating arm 125 is pivotally supported at 125*, whereby movements of this cam follower are transmitted to the valve stem 51 and, of course, correspond to the contour of the cam plate.

The damper 52 is actuated by a link 126 connected to the bolt 126 Fig. 3, attached to the free end of a pivoted arm 127 which carries a cam follower in the form of a roller 128 bearing against the cam surface 121 of cam plate 121. The movements of the latter cam follower are transmitted to the damper and, obviously, correspond to the contour of cam surface 121*.

The two cam-carrying anns 122 and 123 are formed integrally with a hub 130 which is keyed to shaft 88 and supported by sleeve 92 through the medium of a roller bearing 136.

The pistons 114 and 115 of the control unit (Fig. 2), the fuel valve 51 (Fig. 8) and air damper 52 (Fig. l) are shown in their intermediate positions. Also the power movement of hydraulic motor piston 115 toward the left in Fig. 2 represents a movemnet for increasing the supply of fuel and air. It will be observed that cam plate 120 is inclined upwardly and toward the left with respect to the arm 122 in Fig. 10 and cam plate 121 is inclined downwardly and to the right with respect to arm 123 in Fig. 9, therefore a clockwise movement of the cams 120 and 121 (Fig. 8) increases the delivery of fuel and air and a counter-clockwise movement of said cams 120, 121 decreases the delivery of fuel and air. Therefore, in order to maintain a relatively steady movement of the control unit 24 in a direction to increase the delivery of fuel and consequently increase the fire and to insure a quick movement of the unit when it becomes desirable to shut off the fire, a one-way retarder 137 is interposed.

in the. conduit lll'for' deliverin pressureflui-d to cylinder 113'. This'r'etar'der includes a body 138 formed with a cone-shaped seat 139. A ball 1 4i resting on the cone seat 139 blocks the port 141. Consequently, the pressure fluid delivered from conduit 111 into the cylinder 133 must pass throuhg a restricted channel 14-2 formed in the seat 139, and thereby retard the power movement of piston liS. However, when the piston 115 is moved to the" rightin Fig. 2, the pressure fluid expelled from the cylinder 113 unseats the ball 14% of the retarderand therefore permits quick movement of the control unit in a direction to reduce or shut oh" the delivery of fuel.

It will be apparent from the foregoing description that the invention herein disclosed involves an the one hand a novel system and method of control wherein the con trol is predicated upon variations in the rate of flow of feed Water to a boiler or steam generator asdistinguished from control systems functioning in response to ten:- p'e'ratu'r'es. On the other hand, it also will be apparent that the invention includes also a new motion transmitting or translating mechanism which is particularly suitable for use intconnectien with and forms an important part of the system. Accordingly certain of the appended claims are directed to the system in combination with the improved control unit and certain other claims are directed to the control unit per se'.

It will be evident to those skilled in the art that the" new principle of operation underlying the described control unit is susceptible of a considerable variety of em hodiments. It is to be'understood, therefore, that the inventiontcontemplates all modifications in principle and structure that come within the scope of the appended claims.

We claim:

1. A system comprising a liquid heating unit, a conduit for delivering a liquid into said heating unit, a heat generator for supplying heat to said heating unit, a fuel valve for controlling the supply of fuel to the heat generator.

means for variably regulating the rate of delivery of said liquid totsaid heating unit in direct proportion to the fuel delivered and consequently indirectly proportionately to recurrent variations of the pressure generated in the heated'flu'id within the heating unit'so that the rate of delivery of said liquid proportionately decreases as the said'pressure in the heated fluid within the heating unit increases and the rate of delivery of said liquid increases as the pressure in the heated fluid within the heating unit decreases, and a perforate element positioned in said liquid delivery conduit and movable in response to flow or" liquid therethrough and directly proportionally with the flow rate thereof, said element being operatively connected with the fuel valve to proportionally control the delivery of fuel to the heat generator in direct proportion to the rate of flowof said fluid to the heating unit. V t

2. A system comprising a steam generating unit, a water feed conduit leading into said unit, a fluid fuel burner for supplying heat to said unit, a fuel valve for controllingthe supply of fuel to the burner, a damper for controlling the supply of combustion air to the burner,

means including a feed water control valve structure re-.

sponsive to steam pressure variations in said unit to decrease the rate of flow of a continuous stream of feed water to said unit in relation to increases insteampressure and vice versa, a device including a flexible diaphragm having an orifice therein and interposed in said conduit in a position to be deflected by pressure exerted thereon by the feed water, a pin extending axially through 7 said orifice and tapered to decrease the diameter of' the pin in the direction of flow of the water, whereby the deflection of the diaphrag'm is proportional to the momentary' rate' of flow tof feed water through said orifice to said unit, and mechanism operatively connected to said siaphragn snd to saidfuel valve and said damper to control said fuel valve and said dam er in relation to p of flow of the stream of feed water to said unit and to 75 the extent of-reciirreiit deflections of saiddiaphragm so as to proportionately increase the delivery of fuel and air to the burnerin relation to increases in the rate of now of the stream of feed water to said unit and to propertionately decrease the delivery of fuel and air to said burner in relation to decreases in the rate of feed water now to said unit;

3. A s-yst'rn as defined in claim 2 characterlzed in that j the fuel is delivered under pressure to the burner by means of a fuel pipe and a pump interposed therein, the said mechanism which is operatively connected to the diaphragm and to the fuel valve and the damper is a fluid pressure servomdtor, and a branch pi e leads from said fuel pi e to' saia'ser'vonioter to supply it with fluid under pressure. a V

4.- A system as defined in claim 3 characterized in that said seivoinotor is operable in reverse directions and in that means is provided for retarding its movement in one direction, so as to avoid abrupt increases in the delivery of fuel and air to the burner.

5. A system asdefined 'n claim 4 characterized in that said retarding means for the servornotor comprises a ball valve structure interposed in a fluid pressure line leading to a pressure operated element of said servomotor.

' 6. A system eem rtsjing 'a steam generating unit; a water feed conduit leading into said unit,- a fluid fuel burner for supplying heat to said unit, means including a fuel valve for controlling the supply of fuel to the burner, means for re ulating the rate of flow of a continuously flowing stream of feed water to said unit in inverse proportion to recurrent variations of steam pressure'within the unit so that the rate of flow of the feed water stream decreases as the steam pressure increases and vice versa, a device inelildinga fleiiible diaphragm interposed in said conduit in a position to be deflected by the pressure exerted therein by the feed water and having an orifice therein,- a pin extending hrough said orifice and tapered to decrease the diameter of the pin in the direction of feed water now, whereby the deflection of the diaphragm is directly proportional to the rate of flow of the feed water to said unit, and rheehanis'ni operatively connected to said diaphragm and to said fuel valve to control said fuel valve in relation tot-he extent of the recurrent deflection of said diaphragm so as to proportionately increase the delivery of'fuel'to the burner in relation to increases in 7 a diaphragm interposed in said conduit having an orifice therein for restricting the passage of water and deflectable by pressure exerted thereon by the feed Water, means for proportionately diverting feed water from the upstream portion of said conduit relative to said diaphragm including means defining a bleed port for discharging feed water from the conduit, a valve structure responsive to a predetermined steam pressure within the steam generating'u'nit for controlling the effective opening of said bleed port, whereby the rate of flow of feed Water to said unit and the deflection of said diaphragm is regulated'in' inverse proportion to the recurrent variations of steam pressure within said steam generating'unit, a valveifor controlling the supply of fuel to said burner, and mechaa nism'operatively connected with said diaphragm and'with said fuel valve to control the'delivery of fuel to said unit 7 in rel tion tothe extent of recurrent deflection of said diaphragm seas ito proportionately increase the'delivery of fuel to the burner in relation to increases in the rate roportionately decrease the delivery of fuel to said burner in relation to decreases in the rate of feed water flow to said unit.

8. The method of controlling a continuously operable liquid heating unit supplied with a heating medium and through which a liquid to be heated is passed in a continuous stream in heat exchanging relation to said heating medium, which method comprises utilizing the flow of said liquid into said unit to actuate a movable element to an extent directly proportional to variations in the flow of said liquid, utilizing the movements of said element to directly proportionately control the amount of heating medium supplied to said heating unit to heat the said liquid sufiiciently to provide vapor pressure within the heating unit, and utilizing said vapor pressure to vary the rate of flow of said fluid into said heating unit so as 10 to increase the rate of flow of fluid into the heating unit as said vapor pressure in said unit decreases and vice versa.

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